136 University of California Publications. [Zoology 



legs. In the male a peculiar brush-like organ is found between 

 or in front of the first pair of legs ; this organ is never present in 

 the female. Furca always small and weak, usually with two setae. 



Genus Xestoleberis Sars, 1865. 



Shell smooth, left valve somewhat larger than right so that 

 its anterior, ventral, and posterior margins overlap the right 

 valve when closed ; a tooth at the anterior end of the hinge and 

 one at the posterior end. 



Antennule six-jointed, only moderately elongated ; armed with 

 regular setae. Antenna four-jointed (3 and 4 fused) ; last joint 

 armed with two strong claws. Basal joint of mandible short; 

 mandibular palp short, three-jointed (1 and 2 fused) ; with a dis- 

 tinct respiratory plate which bears two respiratory setae The 

 respiratory plate of the maxilla bears a long- aberrant seta which 

 is not plumose. The three pairs of legs moderately elongated ; 

 anterior and posterior margins of the first joint of the legs with 

 at least oue seta in addition to the claAV-like setae at the distal 

 ends of these joints. In the female the furca ahvays bears two 

 setae, of which the upper is the longer than the lower; in the 

 male the furca is more or less closely united with the penis, and 

 sometimes ))eai"s three setae. The penis is always distinctly sepa- 

 rated into a basal portion and a movable head portion. 



Xestoleberis dispar Miiller. 



PI. 18, figs. 1, 2. 

 1894. Xestoleberis dispar, Miiller, G. W., p. 334, pi. XXV, figs. 2, 3, 9, 35. 



Height of shell of female more than half the length, about 

 1 :1.7 ; highest just behind the middle ; dorsal margin broadly and 

 regularly arched ; posterior margin boldly arched, the boldest 

 portion of the arch lying just above the middle; ventral margin 

 curving in somewhat anteriorly, making a rather bold arch at the 

 antero-ventral corner ; anterior margin regularly curved. Width 

 about equal to greatest height, sometimes a little greater than 

 height. 



Shell of male in side view nearly oval in outline; scarcely 

 twice as long as high; dorsal margin regularly arched, ventral 



